Abstract:

A process variable transmitter that preferably includes a transmitter
output circuit that provides bidirectional HART and controller area
network communication transceiver lines. The transmitter output circuit
also includes sensor circuit interface contacts. An isolated circuit
couples to the sensor circuit interface contacts. The isolated circuit
includes sensor circuitry sensing a process variable. The isolated
circuit further comprises a galvanic isolation barrier galvanically
isolating the sensor circuitry from the HART and controller area network
transceiver lines. A stacked power supply provides power management.
Other aspects may include a controller area network current limiter
diagnostic output, timed sequencing of microcontroller startup and
shutdown, a local operator interface and power management.

Claims:

1-18. (canceled)

19. A process variable transmitter, comprising:a microcontroller energized
by a first power supply rail;an EEPROM circuit storing controller area
network configuration data received from the microcontroller; anda
controller area network circuit energized by a second power supply rail,
and receiving the controller area network configuration data from the
microcontroller; and the energization of the first power supply rail is
sequenced to fall after the energization of the second power supply rail
when the transmitter is de-energized.

20. The process variable transmitter of claim 19 wherein the first power
supply rail is energized before the second power supply rail when the
transmitter is energized.

22. The process variable transmitter of claim 19 wherein the controller
area network circuit includes a KEYS circuit that interrupts CAN
communication when keys of a local operator interface are pressed.

23. The process variable transmitter of claim 19 wherein the
microcontroller is energized at startup before the controller area
network circuit.

24. The process variable transmitter of claim 23 wherein the
microcontroller remains energized at shutdown after the controller area
network circuit is de-energized.

25-27. (canceled)

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]The present application is a continuation-in-part of and claims
priority from U.S. patent application Ser. No. 10/235,874, filed Sep. 6,
2002 for inventors Steven R. Trimble, Kelly M. Orth, Richard M. Nelson
and David G. Tyson and titled "LOW POWER PHYSICAL LAYER FOR A BUS IN AN
INDUSTRIAL TRANSMITTER," the content of which is hereby incorporated
herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002]Process variable transmitters are used to sense process variables
and provide electrical outputs that represent the magnitudes of the
process variables. As electronic and sensor components in process
variable transmitters are increasingly miniaturized, and more functions
are added to the transmitters, the circuitry inside the transmitter
becomes packed very densely, leading to new power management, noise and
interference problems internal to the transmitter.

[0003]There is a problem with noise affecting low level sensor circuitry
in two-wire HART transmitters that also include a controller area network
(CAN) transceiver line. There is a need to provide better protection from
noise in sensor circuitry in miniature two-wire process variable
transmitters that include a CAN transceiver line.

[0004]There is also a problem with meeting the energization needs of the
CAN circuitry and other transmitter circuitry from the loop energization
which is mismatched to the energization needs.

[0005]Transmitters are needed that overcomes these problems. Embodiments
of the present invention provide solutions to these and other problems,
and offer other advantages over the prior art.

[0021]FIG. 13 illustrates a simplified timing diagram of energization of a
transmitter that includes a CAN circuit.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0022]In the embodiments described below, problems with power management,
noise and interference in high density circuitry in a process variable
transmitter with a CAN transceiver line are alleviated. Low level sensing
circuitry is referenced to process ground and a galvanic isolation
barrier is provided between the sensing circuitry and HART and controller
area network (CAN) transceiver lines. The HART and CAN transceiver lines
are not able to effectively couple noise into low level sensing circuits,
and the transmitter can take advantage of miniaturized components to make
a compact transmitter.

[0023]A stacked power supply allows current through CAN circuitry and
other transmitter loads to exceed a minimum loop current through re-use
of current.

[0024]FIG. 1 illustrates an internal connection diagram of an exemplary
embodiment of an industrial process variable transmitter 100. The process
variable transmitter 100 includes printed circuit boards such as a sensor
board 200, an RFI board 300, an output board 400, and a CAN board 500
that are electrically connected to provide transmitter functions. The
transmitter 100 couples to a fluid process inlet fitting 102 and senses a
process variable such as pressure, temperature, flow or the like, of a
process fluid in the process inlet fitting 102.

[0025]The transmitter 100 includes loop leads 104, 106 for coupling to an
industrial process control loop (illustrated in FIG. 3) that is external
to the transmitter 100. The transmitter 100 may include grounding lead
108 for connecting the transmitter 100 to a process ground (illustrated
in FIG. 3). The transmitter includes a housing 112 that is connected to
process ground by way of the grounding lead 108. The housing 112 may also
be connected to process ground through the process inlet 102. The
transmitter 100 includes a CAN transceiver lead 114 for connection to one
or more CAN devices (illustrated in FIG. 3) that are external to the
transmitter 100.

[0026]The loop leads 104, 106 are bidirectional HART communication
transceiver lines for communication with an external device that is
compatible with HART communications. The CAN lead 114 is a controller
area network transceiver line for communication with an external device
that is compatible with CAN communications.

[0027]In a preferred embodiment, the leads 104, 106, 108, 114 are flying
leads, however, other electrical connection devices such as screw
terminals, compression terminals, multipin connectors and the like can be
used as well.

[0028]The transmitter 100 is a two-wire transmitter in the sense that it
is an electronic transmitter that uses two power wires 104, 106 for
signal transmission. The two-wire transmitter 100 also includes a ground
lead 108 and a CAN transceiver lead 114.

[0029]The CAN board 500 includes contacts J4A-1 through J4A-8 that are
connected by a connector 502 to correspondingly numbered contacts J4-1
through J4-8 on the output board 400. The sensor board 200 includes
contacts J5-1 through J5-8 that are connected by connector 202 to
correspondingly numbered contacts J2-1 through J2-8 on the output board
400. The sensor board contacts J5-1 through J5-8 comprise sensor circuit
interface contacts. The sensor board 200 preferably includes a galvanic
barrier 204 that isolates sensor board circuitry that is connected to
connector 202 from an isolated circuit 201 on the sensor board 200 that
is connected to process ground. The sensor board 200 comprises an
isolated circuit coupled to the sensor circuit interface contacts.

[0030]The output board 400 includes contacts J1-1 through J1-8 that are
connected by a connector 402 to correspondingly numbered contacts J1-1
through J1-8 on the RFI board 300. The connectors 202, 402, 502 can
comprise flexible ribbon cables, in line pins or other know connectors
for making connections between printed circuit boards.

[0031]Examples of circuitry on circuit boards 200, 300, 400, 500 are
described below in connection with FIGS. 2-7, 9-12. It will be understood
by those skilled in the art that the arrangement shown in FIG. 1 is
merely exemplary, and that the number and type of circuit boards in a
transmitter can vary from that shown depending on the particular process
variable that is sensed and the needs of a particular application.

[0032]FIG. 2 illustrates a block diagram of an exemplary embodiment of a
sensor board 200. An isolated circuit 201 on the sensor board 200 is
galvanically isolated from the contacts J5-1 through J5-8. A process
pressure sensor 206 couples to a process inlet 102 for sensing process
pressure. The process inlet 102 typically includes a threaded metal
fitting that connects to process ground 103. There is a need for a fluid
connection between the process inlet 102 and the process pressure sensor
206. There would be a potential for noise coupling between process ground
and the sensor circuitry if the sensor circuitry were to have ohmic
connections to a CAN transceiver line instead of the galvanic isolation
provided. A second pressure sensor 208 preferably comprises an absolute
pressure sensor, and senses barometric pressure or a second process
pressure. While the sensors 206, 208 are illustrated as included in the
sensor board 200, it will be appreciated by those skilled in the art that
the location and number of the sensors can vary depending on the type of
process variable that is sensed, and that sensors can be located on a
circuit board as illustrated, or can alternatively be mounted to a
transmitter housing 112 (FIG. 1).

[0033]Sensors 206, 208 couple to a sensor circuit 210 that energizes the
sensors 206, 208 and provides data on lines 214, 216 and communicates
sensor outputs to a microcontroller 220 over an SPI serial communication
bus that includes lines SCLOCK, MISO and MOSI as illustrated in FIG. 2.
The sensor circuit 210 preferably comprises a multichannel sigma-delta
analog-to-digital converter circuit. The micro-controller 220 preferably
comprises a National Semiconductor COP8SGE728M8 microcontroller that is
programmed to provide communication protocol conversion from an SPI
protocol to an SCI protocol.

[0034]A galvanically isolated power supply 222 energizes isolated
circuitry 201 on the sensor board 200 with supply rails VDDA, VMID and
VSSA and also provides a clock signal 224. The galvanically isolated
power supply 222 includes an isolation transformer (such as illustrated
in FIG. 7) that is part of a galvanic isolation barrier 204 that
galvanically isolates the contacts J5-1 and J5-8, which are indirectly
connected to HART and CAN transceiver lines, from the low level isolated
circuitry 201 on the sensor board 200. The low level circuitry 201 on the
sensor board 200 is connected to the housing 112, which is usually
connected to process ground 103.

[0035]The contacts J5-2,3 on the power supply 222 are connected to
circuitry (on the output board 400) that is referenced to the loop
contacts 104, 106. The galvanic isolation provided by insulation in the
isolation transformer forms an electrically insulating barrier between
contacts J5-2,3 and the process ground 103.

[0036]The clock signal 224 couples to a clock shaper 228. The clock shaper
228 provides a shaped clock output 230 suitable for use as a clock input
of the microcontroller (protocol converter) 220 and sensor circuit 210. A
power on reset circuit 232 provides a power on reset signal 234 to a
reset input of the microcontroller 220.

[0037]The supply rail VSSA is preferably coupled to the housing 112 such
that the transmitter housing 100 and the process inlet 102 are not able
to capacitively couple noise from the environment surrounding the
transmitter 100 into low level circuitry on the sensor board 200. With
this arrangement, the low level circuitry on the sensor board 200 is
effectively surrounded by an electrostatic shield which is the metal
housing 112.

[0038]The microcontroller (protocol converter) 220 couples input and
output data for the sensor board 200 through a galvanically isolated
serial bi-directional communication circuit 236. The circuit 236 includes
an isolation transformer (such as illustrated in FIG. 6) that is part of
the galvanic barrier 204. The galvanic barrier 204 isolates circuitry on
sensor board 200 from the contacts J5-1 through J5-8. With galvanic
isolation, there area no electrical conduction paths in the transmitter
100 between the contacts J5-1 through J5-8 and the low level circuitry on
the sensor board 200. The galvanic isolation barrier permits the low
level circuitry on sensor board 200 to be connected to the grounded metal
housing 112 for noise immunity while the high level circuits in
transmitter 100 are referenced to the loop terminals 104, 106 to avoid
stray ground currents. The galvanic isolation barrier 204 prevents stray
ground currents between the current loop and process ground. Galvanic
isolation can also be accomplished by the use of optical isolators in
place of an isolation transformer. Information and power pass through the
galvanic barrier, however, electric currents are blocked and do not pass
through the galvanic barrier.

[0039]FIG. 3 illustrates an exemplary embodiment of the RFI board 300. The
RFI board 300 couples via a two wire process control loop 301 to a loop
receiver 302 that is external to the transmitter 100. The loop receiver
302 is typically an input channel of a process control system that
provides a current that energizes the transmitter 100. The loop receiver
302 preferably senses a magnitude of a 4-20 mA loop current controlled by
the transmitter 100. The loop current is representative of a corrected,
sensed process variable that is sensed by sensor 206 (FIG. 2). The loop
receiver 302 also preferably exchanges bi-directional HART digital
communications signals with the transmitter 100. These HART digital
communication signals are superimposed on the 4-20 mA current and are in
a frequency range that does not interfere with sensing the 4-20 mA
current magnitude.

[0040]The RFI board 300 also couples to an external CAN device 304. The
external CAN device is referenced to loop minus via line 307 as
illustrated. A pi filter 311 filters out RFI on CAN line 306. The
external CAN device 304 is external to the transmitter 100 and
communicates with the transmitter 100 using a controller area network
(CAN) serial communication protocol. The external CAN device 304 can be
any type of industrial device that can utilize communication with the
transmitter 100. The external CAN device 304 may, for example, comprise a
temperature sensor to provide temperature correction data to the
transmitter 100. The external CAN device 304 may also comprise a local
controller that receives process variable data from the transmitter 100.
The external CAN device 304 may also comprise another transmitter of a
design similar to the design of transmitter 100. The transmitter 100 thus
has a first bi-directional serial communication capability for longer
distance communication using the HART protocol on the process control
loop 301, and also has a second bi-directional communication capability
for shorter distance communication using the CAN protocol on a local two
wire connection on lines 305, 307.

[0041]The RFI board 300 couples communications with the CAN external
device 304 along line 306 to contact J1-8 on the output board 400 (FIG.
1). Communication signals from the CAN external device 402 pass through
line 306 and also a conductor on the output board 400 to connector J4A-3
on the CAN board 500 (FIG. 1).

[0042]Current from the process loop 301 is carried by the RFI board 300 to
the output board 400 (FIG. 1) by way of the contacts J1-2 and J1-3 on the
RFI board 300. The RFI board 300 includes a radio frequency interference
(RFI) filter 308 that filters out RFI and that couples the loop current
from the loop receiver 302 to the contacts J1-1, 3. RFI filter 308 is
designed to allow passage of higher frequency superimposed HART digital
communications, which are in an audio frequency range. Power supply
circuits portions 310, 312 of a power supply circuit are located on the
RFI board 300 in order to provide a direct thermal connection between
portions 310, 312 and the housing 112 for good heat sinking. A ground
connection 309 in the RFI filter is connected to process ground 103 by
way of wiring post WP5. The ground connection 309 is also connected to
the housing 112 by way of wiring post WP14.

[0043]FIG. 4 illustrates an exemplary embodiment of a CAN support circuit
403 on the output board 400. The CAN support circuit 403 operates in
cooperation with circuitry on the CAN board 500 (described below in
connection with FIGS. 5A, 5B) to provide bi-directional communication
supporting a CAN communication protocol for communication with the
external CAN device 304 (FIG. 3).

[0044]The CAN support circuit 403 includes a microcontroller 404. In a
preferred embodiment, the microcontroller 404 comprises an 8 bit
microcontroller type ATMEGA103L. The microcontroller 404, in addition to
controlling the CAN support circuitry 403, also preferably provides
control to the sensor board 200 and portions of the output board 400 that
support the 4-20 mA current output and HART serial communications
outputs.

[0045]The CAN support circuit 403 also comprises a CAN controller 406. In
a preferred arrangement, the CAN controller 406 comprises a type MCP2510
controller from Microchip Technology Inc. of Chandler, Ariz. The CAN
controller 406 receives CAN-formatted communications on a CANRX line 408
and transmits CAN-formatted communications at a TXCAN output 410. In a
gating circuit 412, the TXCAN output 410 is combined with a KEYS gating
output 414 from the microcontroller 404. When active, the KEYS output
indicates that the microcontroller 404 is in a process of receiving
keyed-in configuration information via a local operator interface (LOI).
The gating circuit 412 prevents transmission of CAN formatted
communication to the CANTXO line 416 while a configuration process using
keys on the LOI is underway. An exemplary LOI is described in connection
with FIG. 4 of U.S. Pat. No. 6,484,107 B1 Roper et al. U.S. Pat. No.
6,484,107 B1 Roper et al. is hereby incorporated herein in its entirety.
The LOT is external to the transmitter and can connect to an optional
lead 813 (FIG. 8) for communication with a microcontroller (such as
microcontroller 852 in FIG. 9B).

[0046]The CAN controller 406 communicates data (that does not have CAN
formatting) with the microcontroller 404 by way of an SPI serial
communication bus that includes MOSI line 418 and MISO line 420. The
microcontroller 404 also applies a chip select signal CSCAN to the CAN
controller 406 to control communication with the CAN controller 406. The
CAN controller 406 communicates an interrupt signal CANINT 422 to the
microcontroller 404 when CAN communications are being processed.

[0047]The microcontroller 404 couples a reset signal CANRESET to the CAN
controller 406 at the time that power is applied to the transmitter 100
from the two wire loop.

[0048]FIGS. 5A, 5B illustrate an exemplary embodiment of circuitry on the
CAN board 500. The circuitry in FIGS. 5A, 5B is best understood when
FIGS. 5A, 5B are joined together along dashed lines 520, 522 to form a
single schematic of FIGS. 5A, 5B. In a preferred arrangement, the
circuitry shown in FIGS. 5A, 5B comprises a dominant-recessive CAN driver
arrangement of the type described in the above-cited U.S. patent
application Ser. No. 10/236,874 entitled LOW POWER PHYSICAL LAYER FOR A
BUS IN AN INDUSTRIAL TRANSMITTER filed on Sep. 6, 2002.

[0050]Referring back to FIG. 3, bi-directional CAN signals are
communicated with CAN external device 304 along line 306 to contact J1-8
on the RFI board 300. Contact J1-8 on the RFL board 300 connects to
contact J1-8 on the output board 400 as illustrated in FIG. 1. Contact
JI-8 on the output board 400 connects via. conductor 426 (FIG. 4) to
contact J4-3 on the output board 400. Contact J4-3 on the output board
connects to contact J4A-3 on the CAN board 500 as illustrated in FIG. 5A.
The CAN external device 304 is thus connected through a series of
conductors and contacts to the conductor 504 as illustrated in FIG. 5A.

[0051]In FIG. 5A, protection or clamp diodes 506, 508 are connected to
conductor 504 to limit the voltage on conductor 504 to a range of
approximately +3.7 volts to -0.7 volts. This clamping arrangement limits
noise and does not interfere with the normal levels of CAN communication
signals.

[0052]When the gating circuit 412 (FIG. 4) generates outbound CAN
communications on the CANTXO line, these outbound CAN communications are
conducted through connector J4-5 (FIG. 4) and connector J4A-5 (FIG. 5A)
to line 510 (CANTXO) in FIG. 5A. The circuitry in FIG. 5A amplifies the
relatively low power CAN communications CANTXO signal on line 510 to
provide a higher power level that is coupled along line 512 to line 504
and on to the CAN external device 304. When the CAN external device 304
generates CAN communications that are inbound to the transmitter 100,
then the amplifier 550 in FIG. 5D receives the inbound CAN communications
on line 504 and amplifies the signals to provide the CANRX signal on line
514. The CANRX signal is conducted by connector J4A-4 (FIG. 5B) to
connector J4-4 (FIG. 4) to line 408 (FIG. 4) and provides the CANRX
signal the CAN controller 406.

[0053]In FIG. 5A, the CANTXO signal is applied to an input of an inverter
532. The arrangement of the inverter 532 ensures that leading and
trailing edges are sharply defined, and that the signal at an output of
the inverter 532 has a low impedance. The output of the inverter 532 is
coupled to the dominant driver circuit 580 that includes a MOSFET 538.

[0054]FIG. 6 illustrates an example of a galvanically isolated serial
bidirectional communication circuit adapted for use in the circuit of
FIG. 2. Circuit 600 includes a transformer 602 that includes a first
winding 604 that is galvanically isolated from a second winding 606 by
transformer insulation materials. The insulation material forms part of
the galvanic barrier 204 that galvanically isolates low level isolated
circuitry 201 (FIG. 2) from high level loop-referenced circuitry that
drives the HART and CAN transceiver lines (FIG. 1). Signal coupling
across the galvanic barrier 204 is magnetic rather than electrical in
order to provide electrical isolation.

[0055]A microcontroller 220 (FIGS. 2,6) is coupled to an amplifier 610
that drives winding 604 with a serial communication signal to transfer
out data and commands from the sensor board. A corresponding serial
communication signal is magnetically induced in the winding 606 and
couples the data and commands from the sensor board via shaping network
612 to the output board. The output board is coupled to an amplifier 620
that drives winding 606 with a serial communication signal to transfer
out data and commands from the output board. A corresponding serial
communication signal is magnetically induced in the winding 604 and
couples the data and commands from the output board via shaping network
622 to the sensor board. Communication is thus bidirectional through
circuit 600.

[0056]FIG. 7 illustrates an example of a galvanically isolated power
supply circuit 700 adapted for use in the circuit of FIG. 2. Circuit 700
includes a transformer 702 that includes a first or primary winding 704
that is galvanically isolated from a secondary winding 706 by transformer
insulation materials. The insulation material forms part of the galvanic
barrier 204 that galvanically isolates low level isolated circuitry 201
(FIG. 2) from high level loop-referenced circuitry that drives the HART
and CAN transceiver lines (FIG. 1). Power coupling across the galvanic
barrier 204 is magnetic rather than electrical in order to provide
electrical isolation.

[0057]Transformer 702 received an energization current at contacts J5-3
and J5-2. An RC network 710 is in series with the primary winding 704 to
better match the transformer to its drive at contacts J5-3 and J5-2. The
secondary winding 706 is connected to a bridge rectifier 712. The bridge
rectifier 712 rectifies the output of the secondary winding 706 and
applies the rectified output to a first regulator circuit 714 that
generates a first low level power supply voltage VDDA. VDDA is preferably
less than 5.5 volts to provide low power consumption. VDDA is connected
to a second regulator 716 that generates a second low level power supply
voltage VMID. VMID is preferably less than 2.3 volts. A common conductor
on the sensor board 718 is connected to the bridge rectifier 712, the
regulators 714, 716 and the isolated circuitry 201.

[0058]FIG. 8 pictorially illustrates a process variable transmitter 800.
The process variable transmitter 800 includes a generally cylindrical
main housing body 802. At a first end of the main body 802, a process
inlet 804 includes an external hexagonal surface 806 for gripping with a
wrench and an internal threaded hole 808 for threading onto a process
pipe (not illustrated) that delivers process fluid to the transmitter
800. A grounding screw 810 is provided on the main body 802 for
optionally connecting a grounding wire between the main body process
ground.

[0059]At a second end of the transmitter 800, four flying leads 812 emerge
from a sealed electrical feedthrough connector 814. The flying leads 812
include a LOOP+, LOOP-, CAN and GROUND leads. Flying leads can be
conveniently and economically connected to field wiring by using pigtail
splices, wirenuts and crimped splicing devices.

[0060]The circuit boards inside the housing 802 are sealed from the
surrounding atmosphere. The housing 802 serves as an electrostatic shield
for low level circuits inside the housing 802. In a preferred
arrangement, the housing 802 has a length L that is less than 19
centimeters (7.5 inches) and a diameter D of less than 5 centimeters (2
inches).

[0061]The problem of noise coupling in a densely packed transmitter with a
CAN transceiver output is solved and a compact transmitter housing can be
used.

[0062]FIGS. 9A-9B, taken together, illustrate a simplified diagram of an
embodiment of a process variable transmitter 820 that highlights certain
stacked power supply features in the transmitter 820. FIGS. 9A-9B are
best understood when joined together along dashed line 822 to form a
single diagram of the stacked power supply arrangement.

[0063]The transmitter 820 is energized by connecting transmitter leads
WP1, WP2 to a two wire, four to twenty milliamperes current loop 824 at
the left side of FIG. 9A. The current loop 824 comprises a DC power
supply 826 connected in series with a load resistance 828. The
transmitter 820 functions as a current controller that controls a loop
current ILOOP in the current loop 824 to a current level that represents
a process variable 825 sensed by the process variable transmitter 820. In
a preferred arrangement, the current loop 824 also carries bidirectional
HART communication signals that are superimposed on the loop current
ILOOP.

[0064]The amount of energization that is available from the current loop
824 for energizing the transmitter 820 is severely limited, and is most
severely limited when the loop current ILOOP is at its lower limit of
four milliamperes of loop current. At the lower limit, the total amount
of current flow through the transmitter is 4.000 milliamperes and cannot
be increased (in order to meet transmitter energization needs) because
the current level is used as a representation of the process variable
825.

[0065]In one embodiment, the current loop 824 can be relied on to provide
a minimum voltage at leads WP1, WP2 of no more than about 12 volts under
worst case conditions. The voltage supplied by the current loop 824 at
leads WP1, WP2 is unregulated, however, and can reach levels as high as
about 42.4 volts, depending on variable factors such as regulation of
loop supply 826, resistance of loop wiring and the level of the 4-20 mA
current.

[0066]The electronic circuitry in the transmitter 820 requires power
supply voltages that are regulated for reliable operation. Accordingly,
the transmitter 820 includes a first voltage regulator 830. In order to
maximize the power available to electronic circuitry in the transmitter
820, the first voltage regulator 830 is adjusted to provide the largest
possible regulated voltage output 840 that can be reliably generated from
the minimum unregulated voltage applied at leads WP1, WP2. Taking into
account voltage drops that are used up by RFI chokes 832, 834, reverse
polarity protection diode 836 and current sensing resistor 838, the
largest possible regulated voltage in one embodiment is about 9.5 volts
relative to a current summing node 831, also called RETURN 831.

[0067]Of the 4.000 milliamperes of current available from the current loop
824, only about 3.1 milliamperes of current is available at the first
voltage output 840 of the first voltage regulator 830 in this embodiment.
The remaining 0.9 milliamperes of current are reserved for maintaining an
adequate current through a darlington transistor 842 to ensure that loop
current ILOOP can be modulated by the transmitter 820 to produce a +/-0.5
mA HART signal at a low current of 3.6 mA commonly indicative of low
level alarms. A standard established by NAMUR (Normenarbeitsgemeinshaft
fur Mess- and Regeltechnik der chemischen Industrie) requires that
current on a 4-20 mA loop drop to 3.6 mA or lower to indicate an alarm
condition of a transmitter. When the transmitter is in this alarm
condition, HART modulation can take the current lower by an additional
0.5 mA. The transmitter's power supplies must be in regulation with only
3.1 mA of current in a worst case condition. The transmitter's various
power supply functions need to draw less than 3.1 milliamperes, and then
the transmitter 820 adjusts a current I1 (through the darlington
transistor 842) so that the loop current is at a level in the range of
4-20 milliamperes that indicates the level of the process variable, and
also at a level of 3.6 mA that indicates an alarm condition.

[0068]In one embodiment, the total power available at the first voltage
output 840 is thus about P=VI=(9.5 volts)(3.1 mA)=29.45 milliwatts. This
total power available is adequate, in terms of the number of milliwatts,
to energize a controller area network (CAN) circuit load 844 along with
other transmitter loads such as analog circuit load 846, transformer
drive circuit load 848, modac load 850 and digital and microcomputer
circuit load 852.

[0069]The MODAC 850 is a circuit which combines functions of a MODEM and a
DAC. The MODEM senses HART modulation in the sense voltage at node 831.
The MODEM can also transmit HART modulation to the darlington transistor
842 through an amplifier 811 (FIG. 9A). The DAC converts a digital
representation of the process variable (provided by the microcontroller
(uC) circuit 852) to an analog current for use as an input in the loop
current controller 843.

[0070]It is found, however, that the voltage and current characteristics
(load characteristics) of the transmitter circuit loads are not well
matched to the voltage and current characteristic (supply characteristic)
of the first voltage regulator 830. The transmitter circuit loads require
supply currents that add up to about 4.1 milliamperes, greatly exceeding
the 3.1 milliamperes available from the first voltage regulator 830. The
CAN circuitry load 844, in particular, requires about 0.6 milliamperes
under worst case conditions when the external CAN devices draw 0.5
milliamperes. It will be understood by those skilled in the art that the
particular levels of current mentioned in this applications are merely
exemplary, and that other levels of current can be used in design
variations within the scope of the 4-20 mA standard, the NAMUR 3.6 mA
standard and the +/-0.5 mA HART modulation standard. The supply current
limit is set so as to not disrupt the functioning of the current loop
within the standards.

[0071]The transmitter circuit loads (844, 846, 848, 850) also require
regulated supply voltages typically in the range of about 5.2 to 3.0
volts, much less that the typical 9.5 volts provided by the first voltage
regulator 830.

[0072]There is thus an overall mismatch between the voltage and current
characteristics of the current loop 824 and the voltage and current
characteristics of the transmitter circuit loads 844, 846, 848, 850. The
characteristics of the current loop 824 are fixed by long-standing
industrial instrument standards that are based on the physics of
intrinsically safe circuits and, as a practical matter, can't be changed
significantly. The characteristics of the loads 844, 846, 848, 850 are
fixed by the available low power integrated circuits, and there is thus
no practical opportunity to change load characteristics. This problem is
compounded by the fact that the loop current ILOOP must be sensed, and
feedback provided, in order to ensure that the loop current corresponds
exactly to the process variable 825 that is measured by the transmitter.
In order to provide feedback, the current sense resistor 838 is provided
to sense current I1 (through darlington transistor 842), current I2 (used
to energize first voltage regulator 830) and current I3 (used to energize
a second regulator 854). A loop current controller 843 controls the
current I1 based on feedback so that the loop current ILOOP is at the
correct level to indicate the process variable 825.

[0073]The positive lead WP1 and a negative lead WP2 carry the loop current
ILOOP in and out of the transmitter 820. Inside the transmitter 820, the
loop current ILOOP separates into several current components that include
the first current I1, the second current I2, the third current I3, and a
fourth current I4. Generally, ILOOP=I1+I2+I3+I4 except for certain small
fixed bias currents that can flow, but which do not introduce error in
the loop current because calibration effectively cancels them.

[0074]The transmitter 820 includes the loop current controller 843. The
loop current controller 843 includes the current sense resistor 838. The
current sense resistor 838 carries the first current I1, the second
current I2, and the third current I3. The current sense resistor 838
develops a sense voltage at node 831 that is fed back along feedback line
845 to an input of the loop current controller 843. The loop current
controller 843 controls the first current I1 as a function of both the
process variable 825 (an input provided by the NODAC 850) and the sense
voltage at node 831. The current sense resistor 838, however, does not
carry the current I4. The current I4 bypasses the current sense resistor
and returns directly to the negative lead WP2.

[0075]The transmitter 820 includes the first voltage regulator 830 that
couples to the positive lead WP1 for energization. The first voltage
regulator 830 provides the first voltage output 840. The first voltage
regulator 830 is energized by current I2. Current I2 passes through the
first voltage regulator 830 and return to the negative lead WP2 by
passing through the current sense resistor 838.

[0076]The transmitter 820 includes a second voltage regulator 854 that
couples to the first voltage output 840 for energization. The second
voltage regulator 854 provides a second voltage output 856. The second
voltage regulator 854 is energized by the third current I3. The third
current I3 couples along line 855 from the second regulator 854 to the
current sense resistor 838.

[0077]The transmitter 820 includes a first load 848 (the transformer drive
circuit load 848 which drives a transformer such as shown in FIG. 7) that
draws a first load current 849 that flows between the first voltage
output 840 and the second voltage output 855.

[0078]The transmitter includes a second load that includes the controller
area network load 844, as well as loads 846, 850, 852. The second load
also includes a number of small fixed bias currents that are returned to
common. The second load draws a second load current 858 that flows
between the second voltage output 856 and the COMMON lead that connects
to the negative lead WP2 through the choke 834. The second load current
858 bypasses the current sense resistor 838.

[0079]The second regulator 854 provides only a portion of the second load
current 858 (I4). The first load current 849, after flowing through the
first load 848, also flows through the second load. The current used by
the first load 848 is effectively reused by the second load because the
first and second loads are stacked, or in other words, are in series.
This reuse of the first load current reduces the amount of current that
the second regulator needs to provide. A load current resistor 860
carries the second load current. The second regulator regulates the
voltage at node 862 to a fixed voltage which is preferably 4.3 volts. The
second voltage 856 thus includes two voltage components. The second
voltage 856 includes a regulated component that is typically 4.3 volts.
The second voltage 856 also includes a variable voltage component that
varies as a function of a voltage drop across the resistor 860. The
voltage drop across the resistor 860 thus includes a component that is
proportional to the second load current 858.

[0080]The loop current controller 843 receives the second voltage 856 on
line 857. The loop current controller 843 senses the second voltage 856
to correct the first current I1 for the second load current 858 that
bypasses the current sense resistor 838. The loop current controller 843
thus controls current I1 as a function of the process variable, the loop
current sensed by resistor 838 and also as a function of the second load
current, even though the second load current does not flow through the
resistor 838. The stacking of the first and second loads allows the load
voltages to be added to better match the available regulated voltage. The
stacking of the first and second loads allows a portion of the available
regulated current to flow through both the first and second loads,
effectively reusing the current, and allowing the total load current to
exceed the available regulated current from the loop. The transmitter 820
is thus able to support a controller area network (CAN) load along with
other transmitter loads without exceeding the current and voltage
limitations of the current loop 824.

[0081]FIG. 10 illustrates a CAN current limiter 914 that limits the amount
of current that the recessive driver 582 (also illustrated in FIG. 5A)
can supply to the CAN BUS 504 (also illustrated in FIG. 5A). Reference
numbers and terminology used in FIG. 10 that are the same as reference
numbers and terminology used in FIG. 5B identify the same or similar
features.

[0082]DC power is provided to external CAN devices (such as an LCD) via
the CAN BUS 504 whenever the CAN BUS 504 is in a recessive state (high
level, typically +3 volts). During a dominant state (low level, typically
+1 volt), a bulk capacitor 904 is charged, and then adequate charge is
available for a high current pulse to the CAN BUS 504 once the CAN BUS
504 returns to a recessive state.

[0083]The CAN physical layer power is provided via the current limiter 914
that is designed to limit current that the recessive driver 582 draws
from the supply conductor 906 (CAN VDD) to a fixed limit. In one
preferred embodiment, the fixed limit is 500 microamperes. This current
limiter 914 ensures that an overloaded or shorted CAN BUS 504 does not
force the transmitter outside of its budgeted quiescent current range on
the 4-20 milliampere current loop (such as current loop 824 in FIG. 10).
The current limiter 914 limits direct current available to the CAN BUS
504 to prevent an overload or short on the CAN BUS 504 from creating an
on scale error on the 4-20 mA transmitter current loop. The bulk storage
capacitor 904 stores charge when the CAN BUS 504 is low. When the CAN BUS
504 is high, charge is transferred to an external can device 930 that is
energized by the CAN BUS 504.

[0084]The current limiter 914 comprises an operational amplifier 912. The
operational amplifier 912 is a rail-to-rail input/output (I/O) component
which controls a field effect transistor (FET) 916 to establish the
current limit. The current limiter 914 comprises a sense resistor 918
that senses current flow from the +4.3V rail 926 to the CAN VDD line 906.
The current limiter 914 comprises resistors 920, 922 that form a voltage
divider that establishes a current limit reference to the amplifier 912.
The current limiter 914 is referenced between the line 924 (+3V) and the
line 926 (+4.3V) to ensure an orderly start-up sequence of the
transmitter, as described in more detail below in connection with FIG.
13.

[0085]In order to provide power to an accessory load on the CAN BUS 504 in
an efficient manner, the physical layer stores charge while the CAN BUS
504 is low and transfer charge to the CAN BUS 504 when the CAN BUS 504
switches back high. The bulk capacitor 904 accomplishes this.

[0086]Since capacitor 904 is charged via FET 916 which limits current,
voltage on capacitor 904 drops momentarily when the CAN BUS 504 pulls
high peak current from it. In one embodiment, capacitor 904 is large
enough in value to maintain a 3.0 Volt working voltage during
communication. This ensures that an external CAN device such as an LCD
has sufficient supply voltage to operate. The capacitor 904 is
replenished between communication packets.

[0087]In order to start-up properly when power is first applied or to
recover from a shorted CAN BUS 504, a start up circuit 586 provides an
alternate path that provides current to the CAN BUS 504. To meet this
requirement, a PNP transistor 902 in start up circuit 586 turns on to
provide power to the CAN BUS 504 after the bulk storage capacitor 904 is
fully charged. The startup circuit 586 pulls the CAN bus high at start up
or upon fault recovery after the bus has been shorted to ground. The
startup circuit 586 provides an orderly power up and efficient use of
power by allowing the bulk capacitor 904 to fully charge before providing
any current to the CAN BUS 504. The CAN physical layer turns the
recessive driver 582 off when the CAN BUS 504 is low to conserve current.
This poses a problem at start up or after the bus has been shorted to
ground. Since the bus is low in either of these cases, the recessive
driver 582 will be turned off. Nothing would pull the bus high to start
it up or recover form a shorted condition. The bipolar PNP transistor 902
provides the pull up path to perform this function. The emitter of the
transistor 902 is connected to line 906 (CAN VDD) by way of the resistor
908, the base of transistor 902 is connected to +3V (either directly as
shown or through a resistor) and the collector of transistor 902 is
connected to the CAN BUS 504. In this embodiment, once line 906 reaches
about 3.6 volts, transistor 902 will turn on and source current to the
CAN BUS 504. This creates a 3.6 Volt rail 906 which is sufficient for the
physical layer requirements. Once the rail 906 as at 3.6 Volts, capacitor
904 is fully charged so there is no where to store additional charge. It
is acceptable to supply current to the CAN BUS 504 as a pull up
mechanism.

[0088]If the CAN BUS 504 is loaded by an excessive load 910, current will
flow to ground but line 906 will be fixed at 3.6 Volts. If there is no DC
load on the bus the current will flow through transistor 902 base/emitter
junction and into the +3.0 Volt rail to be reused. An additional benefit
is that the physical layer draws a fixed current at all times so that the
DC power limit circuit is not in a dynamic application and thereby
keeping switched loads associated with the serial bus isolated from the
+4.3 volt internal rail and from the 4-20 mA loop regulation circuitry.
This allows the use of a relatively slow, low power operational amplifier
912.

[0089]An optional diagnostic circuit 932 can be added to the circuit shown
in FIG. 10. The diagnostic circuit 932 couples to CAN power supply
circuitry and provides a diagnostic output 934 that indicates the state
of regulation of the power supplied to the CAN bus on line 504. If there
is an excessive load 910 (such as excessive cable capacitance), the
diagnostic output 934 can alert an operator of the problem. The load is
excessive when it exceeds the 0.5 mA current limit Iset that is set by
the current limiter 914. The diagnostic output 934 preferably couples to
microcontroller that is part of the digital uC circuits 852 in FIG. 9B.

[0090]In a preferred arrangement, the diagnostic circuit 932 comprises a
PNP transistor 936 with an emitter connected to the base of transistor
902, a base connected to +3V and a collector connected to a resistor 938
that couples to DC common. The diagnostic output 934 is connected to the
junction of the resistor 938 and the collector of transistor 936.

[0091]FIG. 11 illustrates power supply circuitry that is adaptable for use
in transmitters such as those illustrated in FIGS. 9A-9B, 10. Reference
numbers used in FIG. 11 that are the same as reference numbers used in
FIGS. 9A-9B, 10 refer to the same or similar features.

[0092]A CAN external device 930 has certain features that can be
programmed or configured at start-up, and that can be re-configured from
time to time during use. CAN configuration data is transferred to the CAN
external device 930 over the CAN BUS 504 to configure the CAN external
device 930.

[0093]CAN configuration data is transmitted over the two wire 4-20 loop
that powers the transmitter using the HART protocol. A modac 850 receives
HART messages that include the CAN configuration data, demodulates the
HART messages, and provides CAN configuration data to the microcontroller
950. The microcontroller 950 transmits the CAN configuration data to an
EEPROM circuit 952 where it is non-volatilely stored as stored CAN
configuration data 954. Once the CAN external device 930 is configured,
then another HART device (typically a process control system) connected
to the two wire 4-20 milliampere loop can communicate with the CAN
external device 930.

[0094]Each time that the CAN BUS 504 as restarted, the microcontroller 950
automatically retrieves the current version of CAN configuration data
from the EEPROM circuit 952 and then uses the CAN circuit 956 to transmit
the current version of CAN configuration data to the external CAN device
930.

[0095]From time to time, there can be momentary power outages
("brown-outs") on the two wire 4-20 mA loop that energizes the
transmitter. If one of these brown-outs occurs while the microcontroller
950 is writing CAN configuration data to the EEPROM 952, the writing of
data may not be completed, and the stored CAN configuration 954 can be
corrupted or obsolete. After this happens, the process control system may
subsequently attempt to communicate with the external CAN device 930
assuming that the CAN external device 930 is currently configured, when
in fact the CAN external device 930 has an obsolete or corrupted
configuration. Malfunction of the control system can result from this
mismatch of assumed and actual CAN configuration data. In order to reduce
the possibility of such a mismatch, circuitry described below is provided
to prevent such a mismatch.

[0096]The +3V power supply rail is provided with an energy storage
capacitor 958 that the +3V supply voltage drops slowly and maintains the
+3V supply long enough to fully complete a write of CAN configuration
data to the EEPROM 952 after the loop energization is removed. A first
regulated voltage 840 (+9.5V) is provided with only a small energy
storage so that the +9.5V supply drops quickly when the loop energization
is removed.

[0097]The +9.5V supply is sensed by a resistive voltage divider that
includes resistors 960, 962. A comparator 964 has a first input connected
to an output 966 of the resistive voltage divider, and has a second input
connected to a fixed reference voltage 968. The comparator 964 compares
the output 966 of the resistive voltage divider to the fixed reference
voltage 968. A comparator output 970 indicates when the loop energization
has been interrupted. The comparator output 970 couples to a FET 972 that
provides a brown-out output 974 to the microcontroller 950 that is
actuated when loop energization is interrupted.

[0098]When the brown-out output 974 is actuated, the microcontroller 950
responds by taking two actions. In a first action, the microcontroller
950 sets a warning flag, using HART communication, to the process control
system. The flag remains set, and the next time there is a HART
communication, the flag is included in the communication, alerting an
operator that there has been a brownout condition present. The warning
flag alerts the process control system to the possibility that the CAN
configuration data may have been corrupted by a brown-out. After power is
restored, the process control system responds by repeating transmission
of CAN configuration data. In a second action, the microcontroller 950
responds by delaying other microcontroller tasks and quickly completing
writing of stored CAN configuration data 954 while the +3V supply is
still available due to the energy storage in capacitor 958. These two
actions ensure that the configuration of external CAN devices is not
obsolete or corrupted during a brown-out.

[0099]FIG. 12 illustrates power supply circuitry that is adaptable for use
in transmitters such as those illustrated in FIGS. 9A-9B, 10, 11.
Reference numbers used in FIG. 12 that are the same as reference numbers
used in FIGS. 9A-9B, 10, 11 refer to the same or similar features.

[0100]The digital and microcontroller circuits (such as digital and
microcontroller circuits 852 in. FIG. 9B) in a transmitter draw currents
from a +3V power supply that include relatively large current spikes.
These relatively large current spikes can cause instability in the output
of the +3V voltage regulator circuit 1000. A current spike from one
circuit can act as a noise input on other circuits connected to the +3V
supply. In particular, the microcomputer 950, the EEPROM 952 and Hall
effect switches 1002, 1004 tend to generate noise spikes.

[0101]The +3V regulator 1000 provides a regulator output that is coupled
by resistor 1006 to the +3V bus. The resistor 1006 is typically about 10
ohms. The +3V rail is bypassed by an energy storage capacitor 1008. The
energy storage capacitor 1008 is typically about 22 microfarads. The
arrangement of the resistor 1006 and the capacitor 1008 form an RC low
pass filter that tend to detune or decouple the regulator 1000 from its
load on the +3V bus. The arrangement with the RC filter tends to improve
the stability of the regulator output.

[0102]The +3V bus is coupled to the microcontroller by a low pass RC
filter that comprises resistor 1010 and capacitor 1012. The resistor 1010
is typically 150 ohms and the capacitor 1012 is typically 1 microfarad.
This arrangement tends to isolate the +3V bus from noise spikes generated
by the microcontroller 950 and vice versa.

[0103]The +3V bus is coupled to the EEPROM 952 by a low pass RC filter
that comprises resistors 1014, 1016 and capacitor 1018. The resistor 1014
is typically 270 ohms and the capacitor 1018 is typically 47 microfarad.
The resistor 1016 is typically 47 ohms and limits current to the
capacitor 1018. This arrangement tends to isolate the +3V bus from noise
spikes generated by the EEPROM 952 and vice versa.

[0104]The +3V bus is selectively coupled to the Hall effect switches 1002,
1004 by FET 1020. The microcontroller 950 actuates an output SWSTART on
line 1022 to turn on the FET 1020 and couple the +3V bus to a line 1024.
RC networks 1026, 1028 couple the energization on line 1024 to the Hall
effect switches 1002, 1004. The Hall effect switches 1002, 1004 can be
actuated by a handheld magnet in order to manually set span and zero
setting for the loop current. The Hall effect switches 1002, 1004 tend to
draw large current spikes from the power rail upon actuation.

[0105]FIG. 13 illustrates a simplified timing diagram of energization of
transmitter circuitry, such as the transmitter circuitry illustrated in
FIGS. 9A, 9B, 11, 12. In FIG. 13, a horizontal axis 1050 represents time
and vertical axes represent whether full energization is present for each
of the signals represented. A "high" level indicates that a signal has
reached a full energization level, and a "low" level indicates less than
full energization.

[0106]The timing diagram illustrates sequencing of full energization of
supply rails in a transmitter so that distribution of energy during start
up and shut down is biased toward energizing a microcontroller early
during start up and also biased toward de-energizing the microcontroller
late during shut down. The microcontroller includes a software "boot up"
sequence that is longer than the start up sequence for other circuits in
the transmitter.

[0107]It is important to get the microcontroller controller booted up and
controlling a MODAC before the 4.3 V power supply is up to its full
value. This arrangement avoids having the loop current controller draw an
initial spike of overcurrent that could be misinterpreted by the external
control loop as an alarm signal.

[0108]It is also important for the microcontroller to complete storage of
CAN configuration data when there is a brown-out or energization is
removed. As explained above in connection with FIG. 11, collapse of the
first output (+9.5V) signals the microcontroller to complete storage of
CAN configuration data, and the large capacitance 958 keeps the
microcontroller 950 operating long enough to complete storage before the
+3V rail drops so low that the microcontroller can no longer work.

[0109]A sequence is illustrated for a process control loop that is
initially off, then brought to full energization at 1052 and then drops
below full energization at 1054. The transition at 1054 is an example of
a brown-out or of a disconnect of loop energization.

[0110]When the loop is first energized at 1052, the regulator which
supplies energization to a microcontroller (for example, the regulator
1000 in FIG. 12 or the +3V regulator in FIG. 9B) charges relatively large
capacitances (for example, capacitors 1008, 1012, 1018 in FIG. 12) so
that the input supply to the regulator (for example, +4.3V rail in FIGS.
9B, 12) is heavily loaded. The output of the +3V regulator goes into
regulation at 1056 in FIG. 13. Next, in sequence, the +4.3V power to the
CAN circuit and analog circuits at 1058, 1060. Lastly, the first
regulated voltage (+9.5V) goes into regulation at 1062.

[0111]When full power to the loop is lost at 1054, the +9.V rail drops
quickly and the microcontroller receives a brownout flag at 1064
signalling the microcontroller to complete any ongoing CAN configuration
storage. The circuitry shown in FIG. 11 illustrates how such a brownout
flag is generated. Next in sequence, the 4.3V rail falls and CAN
circuitry and analog circuitry lose energization at 1066, 1068. Finally,
the microcontroller loses its energization last at 1070.

[0112]Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize that
changes may be made in form and detail without departing from the scope
of the invention.